FIG. 1 illustrates a portion of a geographic area in which wireless services are provided by a first service provider having a first network of cells 10 and a second service provider having a second network of cell 20. Each of the cells in the first and second networks of cells 10 and 20 represents a coverage area for a corresponding base station BS in the network run by the respective service provider. A base station BS generally provides wireless communication services for mobile stations 12 within its coverage area.
As will be appreciated, the communication between one base stations BS and a mobile station 12 has great potential for interfering with the communication between another base station BS and another mobile station 12. This potential exists for base stations BSs in the same network as well as between base stations BSs in different networks. Also, the communication between the same base station BS and two different mobile stations 12 has the potential for interfering with one another. To avoid this interference, wireless communication systems such as CDMA 2000 wireless systems use codes to code and differentiate transmissions. As will be described below, base stations BSs use codes generated from different offset values to differentiate between transmissions by different base stations. Also, a base station BS uses codes to differentiate between different traffic channels assigned to mobile stations by the base station BS.
In a CDMA 2000 system, each service provider assigns an identifier BS_ID to the base stations BS within its network to uniquely identify each base station BS in the network. However, a base station identifier BS_ID does not necessarily uniquely identify base stations BSs in different networks. When transmitting, a base station codes its transmission based on a different offset such as the PN offset in CDMA 2000. In CDMA 2000, the PN offset is a 9 bit string that differentiates one base station BS from its neighboring base stations BSs even when those base stations BSs are in different networks. As illustrated in FIG. 1, base stations BSs from different networks may be neighboring base stations. While FIG. 1 illustrates neighboring base stations as having coverage areas that are adjacent or near one another (e.g., separated by one or more cells), it will be understood, that neighboring base stations may also have overlapping coverage areas. At a mobile station 12, the mobile station 12 obtains the PN offset from the base station BS to decode the transmissions from the base station BS and differentiate the transmission of one base station BS from another base station BS. In CDMA 2000, for example, the mobile station 12 obtains the PN offset as well as the base station identifier BS_ID from a message broadcast by the base station over a forward control channel.
When a base station BS assigns a traffic channel over which the mobile station 12 may communicate, the base station BS uses codes to differentiate one traffic channel transmission from another traffic channel transmission. In CDMA 2000, a different Walsh code is used to code each traffic channel of a base station BS. As with the PN offset, the Walsh code assigned to a traffic channel between the base station BS and a mobile station 12 is communicated to the mobile station 12 in a message over a forward control channel. At the mobile station 12, the mobile station 12 decodes the transmission over the traffic channel and differentiates one traffic channel from another using the Walsh code.
Codes are also used to differentiate reverse link transmissions (mobile station to base station) over traffic channels. The code used by a mobile station to code and differentiate traffic channel transmissions is referred to as a code mask. In CDMA 2000, the code mask is referred to as the long code mask, and two types of long code masks are known: private and public. In CDMA 2000, the public long code mask (PLCM) is typically formed using the electronic serial number (ESN) of the mobile station 12. FIG. 2 illustrates an example public long code mask formed according to CDMA 2000 that is 42 bits long; however, other long code mask sizes exist. As shown, the long code mask includes 2 bits indicating the type of the long code mask (private or public), 8 bits providing signaling information such as the method used to form the long code mask, and the 32 bit ESN of the mobile station 12. The 32 bit ESN is often considered the variable portion of the PLCM.
For the base station BS to decode the traffic channel transmission for the mobile station 12, the BS must obtain the mobile station's ESN, and numerous methods are known in the art for accomplishing this. As an alternative to the above-discussed PLCM generation method, CDMA 2000 also provides that the base station BS may assign a PLCM of its choosing to a mobile station 12 during a call activation (e.g., call origination or call delivery).
Presently, the supply of 32 bit ESNs is being exhausted, and discussions have begun on transitioning from the use of 32 bit ESNs to 56 bit mobile equipment identifiers (MEIDs). The use of a 56 bit MEID poses several challenges such as in the generation of public long code masks. Directly using the MEID would require hardware changes. Accordingly, several proposals exist for using the MEID to generate the public long code mask that do not require such hardware changes. One proposal is to map the 56 bit MEID to a 24 bit value, concatenate a fixed 8 bit value to the 24 bit mapped value and create a 32 bit pseudo-ESN. The pseudo-ESN could then be used as the ESN in the conventional public long code mask generation process.
However, it has been shown that this pseudo-ESN method does not result in a sufficient number of different public long code masks to prevent an undesirable number of collisions between mobile station communications. A collision is where the transmissions from two or more mobile stations are not uniquely identifiable.
Other techniques attempt to resolve this collision problem by introducing additional complexity into the system. For example, one proposal is to have the base stations assign the public long code mask using a methodology to ensure a unique value within the network. However, during inter-network hand-offs, this uniqueness can not be ensured and collisions may result. In view of this, even further layers of complexity in preventing collisions have been suggested.